There are a host of diverse applications requiring alloys which manifest a desired combination of properties for use under elevated temperature conditions. And nickel-chromium alloys of various chemistries are conventionally used to meet such requirements. In this connection, there are a number of industrial and/or commercial applications in which a material is subjected to repetitive stress. This focuses attention on the properties of low cycle and thermal fatigue. Low cycle fatigue (LCF) can be considered as a failure mode caused by the effect of an imposed repetition of mechanical stress. Thermal fatigue can be considered a form of low cycle fatigue where the imposed repetitive stress is thermally induced as the result of differential expansion or contraction during a change of temperature in the material.
Bellows and recuperators might be mentioned as examples where LCF plays a significant role. High temperature bellows are used to allow passage of hot process gas between different equipment, vessels or chambers where cyclic or differential temperatures may exist. Bellows often have a corrugated structure to permit easy flexure under conditions of vibration and cyclic temperature which induce thermal contraction and/or expansion. Seeking optimum performance for bellows requires maximizing low cycle and thermal fatigue and also ductility and microstructural stability. In practice the approach has been to improve such characteristics through grain size control (annealing treatments) and maximizing ductility. But this can result in lower fatigue strength.
With regard to recuperators they are waste heat recovery devices designed to improve the thermal efficiency of power generators and industrial heating furnaces. More specifically a recuperator is a direct type of heat exchanger where two fluids are separated by a barrier through which heat flows. Nickel-chromium alloys, inter alia, are a preferred common material of construction because of their high heat conductivity, given that waste heat temperatures do not exceed about 1660.degree. F. (about 870.degree. C.). One of the alloys used for this application is the Ni-Cr-Mo-Cb-Fe alloy described in U.S. Pat. No. 3,160,500 ('500) and generically known commercially as Alloy 625.
Among the causes of failure of a recuperator is low cycle and thermal fatigue, with creep, high temperature gaseous corrosion, and excessive stresses due to thermal expansion differentials being others. A cause of premature failure in respect of the earlier designed recuperators has been attributed to lack of recognition that excessive stresses required allowance for thermal expansion. More recently, failures have involved inadequate resistance to thermal fatigue (and also gaseous corrosion). It is virtually impossible, as a practical matter to eliminate thermal gradients in an alloy. High thermal conductivity will minimize thermal fatigue but will not eliminate existing thermal gradients. It might be added that thermal fatique resistance can also be enhanced by achieving improved stress rupture strength and microstructural stability.
In any case, as will be demonstrated infra nickel-chromium alloys such as described in '500 manifest a propensity to undergo premature fatigue failure in applications of the bellows and recuperator types.